rename green.core.InviteError to InvitationError

- make GreenInvitation.accept_invite() account for DISCONNECTED event

darcs-hash:20090320104640-b2873-30a5e04c942a1d7b03b143cf00a904e4c1ac78c0
add docstring for GreenInvitation.send_invite

darcs-hash:20090320104936-b2873-02e25169a344527bdd018003604ba76cdf961679